1. Field of the Invention
The present invention is directed to a magnetic resonance apparatus having a gradient coil that is connected to an electrical conductor arrangement for electrical supply to the gradient coil.
2. Description of the Prior Art
For energy supply in a magnetic resonance apparatus, gradient coils of a gradient coil system are connected to a gradient feed amplifier via a cable. Magnetic resonance technology is thereby a known technique for acquiring images of the inside of the body of an examination subject. In a magnetic resonance apparatus, rapidly switched gradient fields that are generated by a gradient system are superimposed on a static basic magnetic field that is generated by a basic magnet system. The magnetic resonance apparatus also has a radio-frequency system that beams radio-frequency signals into the examination subject for triggering magnetic resonance signals and that picks up the triggered magnetic resonance signals, from which magnetic resonance images are produced.
Appropriate currents must be set in the gradient coils for generating the gradient fields. The amplitudes of the required currents thereby amount to up to several 100 A. The current rise and decay rates amount to up to several 100 kA/s. The driving voltages for the currents that the gradient amplifier must offer thus amount to up to several kV.
Given the presence of the basic magnetic field on the order of magnitude of 1 T, considerable Lorentz forces that lead to oscillations of the overall gradient coil system act on the electrical conductors of the gradient coils. Lorentz forces likewise act on current-carrying electrical conductor arrangements disposed in the region of the basic magnetic field, for example between the gradient coils and the gradient amplifiers.
U.S. Pat. No. 5,394,086 discloses a connecting cable for an employable gradient coil for a magnetic resonance apparatus, wherein the forward and return conductors of the connecting cable are arranged such that the forces that act on the connecting cable arranged in the static basic magnetic field, given a flow of current in the conductors, mutually cancel. In one embodiment, the connecting cable is fashioned as a coaxial cable.
Gradient coils are often cooled as a consequence of the aforementioned, high electrical powers. For example, German OS 197 21 985 discloses a cooling device for the indirect cooling of conductors of the gradient coils. A flexible cooling conduit that is laid so as to have dense flowpaths and through which a coolant is conducted for cooling the gradient coils is introduced into a cylindrical jacket-shaped radial plane of a hollow-cylindrical gradient coil system that is cast with casting resin. In another embodiment of German OS 198 39 987, a conductor of a gradient coil is directly cooled by a coolant conducted through an inner cooling channel that is surrounded by the conductor as profiled segment conductor.
A cable having a direct forced cooling of an electrical conductor is also known from the book Kabelhandbuch, VWEW Verlag, Frankfurt am Main, 1997, page 61, published by the Vereinigung Deutscher Elektrizitxc3xa4tswerke e. V. The cable has a waveguide in its interior that is surrounded by the electrical conductor. An elimination of the waste heat arising in the electrical conductor when a current flows ensues by conducting a coolant, for example oil, through the waveguide.
German OS 101 08 843 discloses a coolable coaxial conductor with an enveloping protective cladding wherein two tubular conductor elements are guided essentially coaxially inside one another as inner and outer conductors. The walls of this tubular conductor elements are maintained spaced from one another over their length by electrically insulating, temperature-resistant spacers. The inner conductor, the outer conductor and the protective cladding define free spaces through which a fluid, particularly air, can flow.
Further, German OS 195 04 742 discloses a water-cooled coaxial cable for a forward and return line for electrical current. The coaxial cable has a flexible inner conductor, a inner conduit of elastic, electrically insulating material that cylindrically surrounds the inner conductor spaced therefrom, a flexible outer conductor that cylindrically surrounds the inner conduit spaced therefrom, and an outer conduit of elastic, electrically insulating material that cylindrically surrounds the outer conductor spaced therefrom. Water can be conducted through the inner conduit and through the interspace between inner and outer conduit, which are galvanically separated from one another.
Finally, U.S. Pat. No. 3,564,108 discloses a coaxial transmission line having an inner conductor and an outer conductor that is spaced such from the inner conductor so that an air space remains between the conductors. An outer surface of the inner conductor and an inner surface of the outer conductor are thereby thinly covered with a material that exhibits low dielectric losses.
An object of the invention is to provide an improved magnetic resonance apparatus having a gradient coil that is connected to an electrical conductor arrangement for electrical supply such that, among other things, a high electrical power can be transmitted to the gradient coil with a compact structure.
The object is inventively achieved in a magnetic resonance apparatus having a gradient coil that is connected to an electrical conductor arrangement for electrical supply, the conductor arrangement according having an inner conductor and an outer conductor that coaxially surrounds the inner conductor and with a cooling channel for transmission of a coolant that is arranged between the inner and outer conductors.
Compared to a comparable coaxial conductor arrangement without a possibility of cooling between the conductors, a higher electrical transmission power can be realized.